The fine structure of Schizosaccharomyces pombe has been preserved with unprecedented quality, using a Balzer's high pressure freezer and freeze substitution fixation. The resulting EM specimens have allowed us both to analyze the 3-D fine structure of the mitotic spindles in S. pombe and to investigate the structure of specific intracellular organelles, such as the spindle pole bodies and the membrane systems of the ER and Golgi apparatus during mitosis. Using serial thin sections of freeze-substitution fixed material, we have been able to track the spindle microtubules in 3-D and determine both the arrangement of the microtubules and the changes in microtubules arrangement that occur during mitosis. (Ding, R. et al, 1993). We are now fixing various strains of yeast that are mutant in genes required for mitosis in order to use 3-D fine structural analysis to characterize mutant phenotype and to help us understand the function of particular gene products in the cell division process. Seven examples of the mitotic mutant, CUT11, have been reconstructed in 3-D, allowing us to learn that these cells have abberant spindle pole function. None of the Seven cells contained a normal spindle, but the degree of abberation was variable. Two cells had one pole that appeared normal in function, because there was a normal MT bundle attached to it, but the other pole was unattached to the spindle and was floating in the nucleus. The others had a second pole, but it was not on the end of the spindle, it was elsewhere on the nuclear envelope. Characterization of spindle pole body duplication in wild type and mutant cells is now in progress.